This invention relates to a heat transfer recording medium. More particularly, this invention concerns to a heat transfer recording medium and a heat transfer recording method which are effectively employable in a heat transfer recording process utilizing a sublimable dye (heat-transferable dye) , can impart high transferability to the dye when heat transfer recording is conducted, and can produce an image which is excellent in density.
Conventional recording and printing methods are now being replaced by newly developed methods such as an ink-jet printing method and a heat transfer recording method, which can simply produce a mono- or full-colored image at high speed. Of these methods, the most excellent one is a so-called sublimation-type heat transfer recording method utilizing a sublimable dye, which can produce a full-colored image having an excellent continuous gradation, comparable to a photographically obtainable color image.
A heat transfer sheet commonly used with the above-mentioned sublimation-type heat transfer recording method is such that a dye layer containing a sublimable dye and a binder is formed on one surface of a substrate film such as a polyester film, and on the other surface of the substrate film is formed a heat resistive layer which protects adhesion between the heat transfer sheet and a thermal head.
The heat transfer sheet is superposed on an image-receiving sheet having an image-receiving layer comprising a polyester resin or the like so that the dye layer of the heat transfer sheet and the image-receiving layer of the image-receiving sheet can face each other. Thermal energy is then applied imagewise to the back surface of the heat transfer sheet by a thermal head, whereby the dye contained in the dye layer is transferred to the image-receiving sheet. A desired image can thus be successfully produced in the image-receiving sheet.
In the above heat transfer recording process, only the dye contained in the dye layer is transferred to the image-receiving sheet, and the binder remains on the surface of the substrate film of the heat transfer recording sheet. A high-density image can be sharply produced when transferability of the dye is high.
The simplest method to enhance transferability of the dye is to apply a large amount of printing energy when conducting printing. It is, however, clear that the application of a large amount of printing energy requires high printing cost, so that such a method is undesirable. In addition, when a plastic film is used as a substrate film of the heat transfer sheet, there is a limitation on the amount of printing energy applicable to the recording sheet.
The use of a dye having a low molecular weight is another well known method for improving the dye-transferability. However, an image produced by using such a dye is suffered from the problem of low fastness; in other words, the image has low bleed and heat resistances.
To eliminate the above problem, a dye having a high molecular weight has been used. However, this is not a good solution, because such a dye has low transferability and cannot provide a sharp and high-density image.
Accordingly, an object of the present invention is to provide a heat transfer recording medium and a heat transfer recording method, which can produce an image having a sufficiently high density with application of printing energy in a smaller amount than that of printing energy required in the conventional recording methods, and can produce an image having a density higher than the above with application of printing energy in the same amount as applied in the conventional recording methods.